The invention relates in general to sewing machines and in particular to a new and useful rotary hook arrangement for a lockstitch sewing machine.
A sewing machine similar to the invention is disclosed in U.S. Pat. No. 4,137,858 to Stepel et al. In the case of the rotary hook of this sewing machine the bobbin capsule carrier of the bobbin housing has a laterally projecting holding finger which for the rotationally secured retention of the bobbin housing engages between two cams disposed on the underside of the stitch plate near the stitch hole. Associated with the rotary hook is a spring rod clamped fixed, which cooperates with an abutment surface of a cutout in the bobbin capsule. The cutout is located at a distance before the holding finger in the direction of hook rotation. The spring rod is arranged and dimensioned so that the holding finger is approximately centered between the two cams when the sewing machine is running, so that a thread passage gap is formed on both sides of the holding finger. As a result, it is possible both at the beginning and at the end of the looping around the bobbin housing to move the needle thread loop through the rear and then through the front thread passage gap without using a capsule release device and without having to rotate the bobbin housing. During the looping of the needle thread loop the spring rod is merely bent back briefly by the needle thread. As the spring rod has little mass, it offers not appreciable resistance if the spring force is rated correctly.
Although the thread passage gaps are wide enough, after the guiding of the needle thread loop around the bobbin housing, a brief impediment of the thread movement occurs nevertheless during the upward pull-back. This is because the lower edge, turned toward the holding finger, of the front cam projects into the pull-off path of the needle thread loop and briefly retains the upwardly moved needle thread loop, which toward the end of the pull-back spins in part uncontrolled, until it snaps off the cam laterally into the thread passage gap. Due to this retention effect of the front cam, coming into play in particular at high rotational speed, the needle thread tension increased in an undesirable manner.
By German OS No. 33 03 033 to Rampack it is proposed to connect the holding finger which serves to prevent rotation of the bobbin housing firmly with the base plate outside the area of the stitch hole and to let its free end engage radially to the hook axis into a U-shaped cutout in the bobbin capsule. Preferably the holding finger is to be arranged diameterically to the stitch hole. By moving the holding means which serve to prevent rotation of the bobbin housing into a region diametrically opposite the stitch hole, it is now indeed possible to pull the needle thread loop upward unhindered after completed looping around the bobbin housing. However, then an impediment of the thread movement occurs during the guiding of the needle thread loop around the bobbin housing, in that it must squeeze through between the holding finger and the abutment faces of the cutout at the time of its greatest expansion.
Lastly there is known from German Pat. No. 31 02 457 a rotary hook revolving about a horizontal axis which to secure the bobbin housing against rotation is associated with a spring-loaded holding finger arranged on a support and with a cam on both sides of the holding finger. The holding finger and what in the direction of hook rotation is the rear cam span a finger formed at the bobbin capsule carrier. The front cam forms a support for the spring loaded holding finger. As the housing side holding means are arranged in the vicinity of the stitch hole, the pull-back of the needle thread loop is impeded, as in the case of the first named rotary hook, so that consequently one must operate with increased needle thread tension.